2D bash arrays: is this possible?

Question

If I want a 2D bash array, is there any recommended way of doing this for say, a N x M arrangement? On several occasions, I've had to define array1, array2, arrayN each of M elements in conjunction with case statements - but this is very clumsy.

I realise it would be possible to define a single big array X and compute a 1-1 map between X[something] and the [i,j] element - but this seems like a lot of work to do it properly.

Does there exist any set of functions which could do this reasonably efficiently? I realise its a bit like making Bash imitate FORTRAN.

Answer 1

You could always switch to ksh93 (whose syntax bash has borrowed in the first place for most things) which has multidimensional arrays.

var=(
  (a b)
  (c d)
)

Use as:

$ print -r -- "${var[0][1]}"
b

To loop over all the elements:

for i in "${!var[@]}"; do
  for j in "${!var[$i][@]}"; do
    print -r "var[$i,$j]=${var[$i][$j]}"
  done
done

Answer 2

You can emulate multidimensional arrays with one-dimensional associative arrays (the way awk does it [1]), or with indexed arrays, by shifting the index.

The problem is that you cannot iterate through them in an obvious, natural way.

declare -A a  # associative array
i=2; j=3; a[$i,$j]=4; echo ${a[2,3]}

unset a
declare -a a  # indexed array, the default
i=2; j=3; a[i<<16|j]=4; echo ${a[2<<16|3]}

Notice that the indexed arrays in bash are sparse; a[1<<30]=1 will NOT create 1Gi - 1 empty slots, neither in actual RAM, nor in the virtual address space. Also, since bash's indexed arrays are implemented as doubly-linked lists, random access to an element is slower than with associative arrays (which are using the same hash table implementation bash is using for caching the paths of external commands).

[1] Awk automatically turns an expression like a[x,y,z] into a[x SUBSEP y SUBSEP z] -- where the default value of SUBSEP is \x1c (^\), a control character not supposed to appear in keys. GNU awk also has real multidimensional arrays as a non-standard extension. The classical awk multi-dim array emulation is also available in perl ($; => SUBSEP).